A generalized system with which an embodiment of the present invention may be utilized is described with FIG. 1. Particular embodiments of the present invention are then discussed with reference to FIGS. 1-3.
Referring to FIG. 1, a motor 20 supplies motive power to a load 22. The power source supplying AC electrical power to the drive motor 20 in this illustration is assumed to be the nominal 60 Hz single-phase U.S. power line frequency.
The current transformer 24 and its associated termination resistor 26 are means typically used in the art to obtain a replica current on lead 28 that is proportional to the actual current supplied to the motor 20 in driving the load 22. It will also be recognized by those familiar with the art that the foregoing illustrates one representative way of obtaining a motor current signal that has impressed thereon extra frequency components indicative of normal and/or abnormal conditions in the motor driven load 22 and in the motor 20.
The motor current signal on lead 28, then, has various frequency components impressed thereon. Usually, the 60 Hz frequency of the AC power source will be the dominant frequency and the most readily detected. But, as is known in the art, there also exist few or many other frequencies in the motor current depending on the nature, condition, and operation of the load and motor at the time the motor current is sampled and analyzed.
Some of these extra, or additional, frequency components have been detected and used in the past. For example, sometimes they have been used to signal the operating condition of the load, or in other cases, to control the operating speed of the motor or provide some other desired control function. There is a need, however, for a means of detecting even lower level components that are present in the motor current signal for even further diagnosis or control, depending on the particular application.
The present inventors have realized that there are usually many additional load originated frequencies present in the current of operating electric motor driven systems, and that these can be used to provide a wealth of additional diagnostic information that has not previously been observed using prior art circuits. Indeed, the inventors have discovered that the imposed frequencies are the result of a modulation process whereby mechanical load variations impress amplitude and/or phase modulations on the motor load current, and it is their detection by synchronous amplitude demodulation and phase demodulation that most efficiently recovers them from the motor current signal.